1. Field of the Invention
The present invention relates to fire tube boilers having a novel structure of boiler shell.
2. Description of the Prior Art
Conventionally, there have been used fire tube boilers in which a large number of fire tubes with an approximately 100 mm diameter are arranged on the boiler drum. This type of fire tube boiler is so constructed that combustion gas is flowed through within the fire tubes to heat the water surrounding them. Fire tube boilers have as an advantage the capability of generating a large amount of steam (hot water) for their sizes, compared with flue boilers.
However, the above-mentioned fire tube boilers are confronting a problem upon discharge of harmful exhausts such as a nitrogen oxide (NOx). The discharge of these harmful exhausts are of intensely growing importance under the recent years' circumstances that environmental problems are being considered more and more significant, accompanied by further stricter administrative regulations.